<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>盒子塌陷问题</title>
  <style>
.father {
  width:400px;
  height: 400px;
  background-color: purple;
  margin-top: 50px;
  /* border: 1px solid red; */
  /* padding: 20px 0; */
  overflow: hidden;
  
}
.son {
  width: 200px;
  height: 200px;
  background-color: pink;
  margin-top: 20px;
}
  </style>
</head>
<body>
  <div class="father">
    <div class="son"></div>
  </div>
</body>
</html>